(1) Field of the Invention
The present invention relates to the fabrication of integrated circuit devices, and more particularly, to a system of automatic overlay correction in the fabrication of integrated circuit devices.
(2) Description of the Prior Art
Lithography is widely used in integrated circuit fabrication. A material layer is deposited on a wafer. A masking material is formed over the material layer and exposed in a particular pattern to form a mask for etching, for example. The material layer is then etched where it is not covered by the mask. Another material layer is deposited over the patterned first material layer and the process is repeated to pattern the second layer. This process is repeated many times in the fabrication of an integrated circuit device. It is important that the patterns are aligned properly between layers. Alignment marks are used to align each layer to the previous layer. For example, a “box-in-box” alignment mark in a scribe line may be used. The previous layer alignment mark box is larger than the current layer's box. The centers of each box are measured in both the x- and y-directions. The difference in the box centers is the translation error of one layer to the other.
Typically, the engineer will manually feed back the translation to the exposure tool, perhaps weekly or monthly for correction of the overlay; that is, how closely two layers are aligned. This manual process is not a real-time correction process. With this method, alignment performance is unstable. The exposure tool may drift at any time. Different layers may have different topology or the wafer may warp slightly due to prior annealing, and so on. Many factors may affect alignment. It is desired to apply overlay correction in a real-time manner.
U.S. Pat. No. 6,218,200 B1 to Chen et al shows a method of aligning multiple layers in an automatic overlay correction system. U.S. Pat. No. 6,127,075 to Hsu teaches a method for checking the accuracy of a measuring instrument used to measure overlay. U.S. Pat. No. 6,023,338 to Bareket discloses a novel alignment mark pattern and measuring method.